Dolphins suffer temporary hearing loss from Navy sonar, scientists have found, suggesting military activity could be causing the animals to become stranded.

The animals, which communicate using a natural form of "sonar ping", are said to confused and frightened by the much louder man-made noise of military vessels.

However, until now studies have not been carried out into the phenomenon, often because of the difficulty in gaining access to classified information about the movement of military ships and submarines.

Now laboratory studies carried out by scientists at the Woods Hole Oceanographic Institution at the University of Hawaii have found man-made sonar can induce at least temporary hearing loss in dolphins, causing mild behavioural effects.

The paper, published in the Royal Society journal Biology Letters, read: "With several marine mammal strandings linked to naval sonar activities, concerns are rising about the effects of naval sonar on marine mammal physiology and behaviour. In controlled laboratory studies, we show that actual sonar pings can induce at least temporary hearing loss in dolphins."

However, the study found it would have to be repeated, intense exposures to cause hearing loss and mild behavioural changes.

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Alan Knight, Chairman of British Divers Marine Life Rescue, said the paper added to evidence suggesting dolphin strandings are caused by sonar.

He blamed a mass stranding of dolphins in Falmouth last year on military activity in the area.

He said: "If sonar can make dolphins even temporarily deaf it shows how loud it is - imagine the level of noise you would need to make you deaf? So, it is that loud then the animals will be frightened and try to get away from the noise which is causing the stranding."